The present invention generally relates to systems, apparatuses, and processes for removing acidic gases from gas streams, including but not limited to utility and industrial flue gases. The invention particularly relates to the control of free ammonia and ammonium sulfate aerosol in wet flue gas desulfurization processes and apparatuses in which an ammonia-containing scrubbing solution is used to produce ammonium sulfate as a byproduct.
Gas-liquid contactors and absorbers are widely used to remove substances such as gases and particulate matter from flue gases produced by utility and industrial plants. Often of particular concern are sulfur dioxide (SO2) and other acidic gases produced by the combustion of fossil fuels and various industrial operations. These gases are known to be hazardous to the environment and their emission into the atmosphere is closely regulated by clean air statutes. The method by which these gases are removed with a gas-liquid contactor or absorber is commonly referred to as wet flue gas desulfurization (WFGD).
The cleansing action produced by gas-liquid contactors and absorbers is generally derived from the passage of gas through a tower cocurrently or countercurrently to a descending liquid that absorbs the targeted gas(es) and particulate matter. Wet flue gas desulfurization processes have typically involved the use of an alkaline scrubbing liquid, such as a calcium-based slurry or a sodium-based or ammonia-based solution. While effective, gas-liquid contactors and absorbers utilizing calcium-based slurries produce large quantities of wastes or gypsum, the latter having only nominal commercial value. In contrast, ammonia-based scrubbing processes have been used in the art to produce a more valuable ammonium sulfate byproduct that is usable as a fertilizer. In these processes, sulfur dioxide is absorbed from flue gases with an ammonium sulfate solution, after which the sulfur dioxide is reacted with oxygen and anhydrous or aqueous ammonia injected into the solution to form additional ammonium sulfate solution or ammonium sulfate crystals ((NH4)2SO4). Particular examples of such processes are disclosed in U.S. Pat. Nos. 4,690,807, 5,362,458, 6,187,278, 6,277,343, 7,771,685, and 9,327,234. In addition to being required to react with sulfur dioxide to produce ammonium sulfate, ammonia also serves to increase the efficiency of sulfur dioxide removal by reducing the acidity of the ammonium sulfate solution, which becomes more acidic with the absorption of sulfur dioxide.
The use and addition of anhydrous or aqueous ammonia to control sulfur oxide gases can result in undesirable levels of ammonia slip. As used herein, ammonia slip refers to free ammonia (anhydrous ammonia, NH3) entrained in a scrubbed flue gas exiting a gas contactor or absorber. In addition to incurring an economic loss because of lost ammonia, free ammonia in the scrubbed flue gas reacts with uncaptured sulfur dioxide and trioxide to create an ammonium sulfate aerosol that may be visible as a blue or white plume in the stack discharge, leading to secondary pollution problems. Controlling the amount of free ammonia in the desulfurization process is in part a function of the ammonia vapor pressure, which results from a combination of pH and levels of unoxidized ammonium sulfite produced by the reaction of sulfur dioxide and ammonia in the absence of sufficient oxygen. High pH values result in high ammonia vapor pressure, which promotes ammonia slip. High levels of unoxidized ammonium sulfite also promote ammonia slip.
FIGS. 1 and 2 schematically represent a flue gas scrubbing apparatus 10 that is disclosed in U.S. Pat. No. 6,187,278 as effective to reduce ammonia slip. As shown, the apparatus 10 includes an upright absorber 12 that is supplied with flue gas through an inlet duct 14. The apparatus 10 operates in a manner that causes absorption of sulfur dioxide from the flue gas using a scrubbing liquid. The scrubbed flue gas that leaves the absorber 12 can be delivered to a stack (not shown) or other suitable equipment through an outlet duct 20. The source of the flue gas may be any process involving the combustion of fossil fuels or various industrial operations in which undesirable gases or particulate matter are produced.
U.S. Pat. No. 6,187,278 discloses the apparatus 10 as utilizing an ammonia-rich scrubbing solution 22, such as an aqueous ammonium sulfate solution containing free dissolved ammonia as the reagent for the desulfurization process. FIG. 1 shows ammonia (NH3) being delivered from a source 32 to a reaction tank 18 via a pump 26, conduit 28, and injection system 30 that comprises multiple spargers 34 that extend across the entire diameter of the tank 18. A recirculation pump 40 serves to recycle the scrubbing solution 22 from the tank 18 through a conduit 16 to a contactor region of the absorber 12, where the solution 22 is introduced through a number of nozzles 24 or other suitable devices. The scrubbing process involves spraying the scrubbing solution 22 into the absorber 12 so as to provide intimate contact between the solution 22 and the flue gas. As a result, the solution 22 absorbs sulfur dioxide and other acid gases, such as hydrogen chloride (HCl) and hydrogen fluoride (HF), if they are present in the flue gas. The solution 22 then falls into the reaction tank 18, where the absorbed sulfur dioxide reacts with the ammonia and is oxidized to form ammonium sulfate. Specifically, sulfur dioxide reacts with ammonia to form ammonium sulfite ((NH4)2SO3) and ammonium bisulfite (NH4HSO3), which are oxidized in the presence of sufficient oxygen to form ammonium sulfate and ammonium bisulfate (NH4HSO4), the latter of which reacts with ammonia to form additional ammonium sulfate. A portion of the scrubbing solution 22 and/or ammonium sulfate crystals that form in the solution 22 can then be drawn off to yield the desired byproduct of this reaction. A sufficient amount of ammonium sulfate may be removed from the scrubbing solution 22 prior to delivery to the absorber 12 in order to maintain ammonium sulfate at a desired concentration in the solution 22.
U.S. Pat. No. 6,187,278 teaches that the manner in which ammonia is injected may promote high levels of ammonia slip, such that ammonia and possibly ammonium sulfate aerosol are discharged into the atmosphere with the scrubbed flue gas exiting the absorber 12. As a solution to this problem, U.S. Pat. No. 6,187,278 injects ammonia into the scrubbing solution 22 in the reaction tank 18 in a dilute form (for example, a dilute aqueous solution) and through the spargers 34 shown in FIGS. 1 and 2, which uniformly disperse the dilute ammonia in the scrubbing solution 22 to reduce the likelihood that pockets of high pH and high ammonium sulfite levels will be present in the scrubbing solution 22, such that more uniform and desirable pH and ammonium sulfite levels are achieved that promote absorption of ammonia and control ammonia slip in the absorber 12. As represented in FIG. 1, the ammonia injected into the scrubbing solution 22 is diluted with oxygen from a suitable source 38, and the resulting mixture is then delivered to the tank 18 via the spargers 34 of the injection system 30. Circulation of the injected ammonia and oxygen in the reaction tank 18 is shown in FIG. 1 as promoted by a fan 42.